1. Field of the Invention
The present invention relates to a method and an apparatus for the selective determination of an isotope portion of a measuring gas by means of non-dispersive infrared spectroscopy. The method includes sensitizing in a first ray path to the isotope component and sensitizing in a second ray path to the isotope-pure measuring gas, and electronically amplifying the measurement results.
2. Description of the Related Art
The selective determination of isotope portions has gained particular importance in the fields of biology, geology and archeology, and in recent times even in the field of medicine. The so-called .sup.13 C-Method, which has been established for a long time in the fields of geology and archeology, is utilized as a method for the determination of the age of dead matter. In addition, stable isotopes have been used for considerable time as marking in order to be able to examine metabolic processes in living matter, for example, in the field of medicine.
When using isotopes in the fields of medicine or biology, i.e., in living matter in general, it is important that stable isotopes are used. In many cases, they are the nitrogen isotope .sup.15 N or the carbon isotope .sup.13 C. Instabile isotopes, i.e., radioactive isotopes, locally introduce energy because of the radioactive decay. This energy changes the metabolic processes or breaks up chemical bonds. In the field of medicine, the use of radioactive isotopes is generally prohibited. The use of .sup.13 C has become popular as a marking element for the diagnostic evaluation of metabolic diseases or diseases of the gastrointestinal system in humans. Particularly successful is the use of .sup.13 C as a marking element for the diagnostic evaluation of infectious stomach diseases, such as, helicopacter pylory infections. These infections can be diagnosed simply after a dose of .sup.13 C-marked urea through the expiration air of patients. This makes a complicated gastroscopy superfluous.
In the above-described applications as well as in other applications, mass spectrometric methods are used because of the requirements made of the selectivity of the measuring method. Because of the necessity to produce a high vacuum and because of the complicated sample preparation as well as the technically complicated design of the analyzer, these methods are expensive and, thus, are difficult to realize in many laboratories.
It is also known in the art to use a purely optical method, i.e., the non-dispersive infrared spectroscopy for the selective isotope portion determination, particularly in the determination of the .sup.13 C/.sup.12 C ratio. Such a possibility has already been described in Chemical Abstracts, Vol. 90, 1979, No. 90:37 27 6d, Helge et al. The use and the conditioning of non-dispersive infrared spectroscopy is also known from U.S. Pat. No. 4,289,347. These known apparatus and methods have the disadvantage that the selectivity or the sensitivity of the infrared spectrometer does not always make possible a secure measurement or a secure diagnosis.